According to Lieu, the gravity needed to hold some galaxies or clusters together might come from “shell-like topological defects.” 

Topological defects are unique compact structures in space that have a high density of matter. 

Such defects likely first occurred in the early universe during phase transition — an event during which matter throughout the universe goes through a major physical change.

These defects might appear as long, linear formations called cosmic strings, or as flat, shell-like shapes.

“The shells in my paper consist of a thin inner layer of positive mass and a thin outer layer of negative mass; the total mass of both layers — which is all one could measure, mass-wise — is exactly zero, but when a star lies on this shell it experiences a large gravitational force pulling it towards the center of the shell,” Lieu explained.

It is somewhat similar to how photons, which themselves do not have mass, still experience gravity due to the presence of big astronomical entities. This is because when gravity warps space and time, it interacts with everything within the curvature whether it has mass or not.